This invention relates to modulators and demodulators and, more particularly, to a universal modem which is configurable to process one of a number of modulation formats.
In recent years due to the rapid development of multimedia, digital, audio and video communication applications, there is a necessity for efficient and reliable signal modulation and demodulation techniques to accommodate different modulation formats chosen based on the media over which the signals are to be transmitted.
For instance, with respect to high definition television, and in fact other types of advanced television transmissions, program material in video, data or audio form is formatted by a channel encoder so as to be transmittable by Vestigial Side Band modulation or VSB which is typical for over-the-air television transmissions. On the other hand, Quadrature Amplitude Modulation or QAM is typically used for cable transmission, whereas Quadrature Phase Shift Keying of QPSK is generally used in satellite communication. Finally, discrete multitone or DMT techniques are typically used for telephone landline signalling such as with Asymmetric Digital Subscriber Lines or ADSL.
It will be appreciated that when a video source at the transmit side is encoded through a source coding unit, the source coding unit is set up to code the video source material in one of the four above-mentioned modulation formats. Likewise, on the receive side, in order for a receiver to decode all of the above-mentioned formats discrete VLSI chips or detectors can be provided in parallel, with the outputs going to source decoding. It will be readily apparent that because there is presently no universal modulation format, receivers destined to be utilized not only for over-the-air transmissions, but also for cable must at least have the capability of decoding the appropriate modulation format. While it is possible to use a parallel brute force approach by merely providing the receive side with a series of demodulators each configured to demodulate a particular format, this approach is costly and error-prone.
As to the parallel approach mentioned above, it is possible to design a modem in which the input signal is fed from the RF-IF stage of the receiver in parallel to each of four demodulators, with the output of each demodulator being switched to source decoding depending on the modulation format of the received signal. Modulation format can be determined in some instances by the channel to which the receiver is tuned, although this can vary from country to country.
For instance, in the United States, for VHF channels 1-13 and UHF channels, VSB modulation is used, with the VSB demodulator switched to source decoding. For the cable channels, generally over Channel 68, QAM modulation is used, with the output of the QAM demodulator switched to source decoding. For satellite channels, QPSK modulation is used, with the QPSK demodulator switched to source decoding, whereas for telephone line usage, assuming a channel indicating landline transmission, it is the DMT output of a demodulator which is switched to source decoding.
This duplication of demodulators aside from being four times the expense of a single demodulator, also has the following problems: First and foremost, it will be appreciated that in order to switch the outputs of the various demodulators to source decoding, the output of the demodulators must have a high impedance to damp switching transients and like anomolies Secondly, care must taken to eliminate cross modulation between the different modulators.
It will thus be appreciated that any parallel processing solution to the problem of multiple modulation formats generally resolves itself into a question of the cost associated with duplicating demodulators and technical complexity which is indeed a factor in mass marketed multimedia receivers for which such demodulator cost duplication and complexity is unacceptable.
On the transmit side, for multimedia transmissions requiring different modulation formats, duplicating modulators are likewise expensive and error-prone in much the same way as described above with respect to demodulators. Also, with specialty services such as data to be provided on different channels, oftentimes the modulation format is different from that of the main transmission. To provide such additional services ordinarily would require separate modulators having the expected cost impact.
Rather than multiplying the number of demodulators and modulators to be able to demodulate the differing modulation formats and to provide for differing modulation formats, in the subject invention, a universal modem is provided which is based on reconfigurable logic for its demodulator as well as its modulator. In one embodiment, a host computer at the receive side detects the modulation type and reconfigures the demodulator in universal modem to demodulate the format of the incoming signal.
In this embodiment in the receive mode, a host computer detects the modulation type of the incoming signal by detecting the channel to which the receiver is tuned and setting the demodulator to the expected format. Thus if the receiver is tuned to a given channel, then the universal modem is reconfigured to the expected format of signals on that channel. This is done on an a priori basis since different groups of channels have different standardized formats.
Alternatively, the host computer can detect the format of the incoming signal by detecting a flag in the header of the incoming data stream indicating modulation type, assuming one is loaded into the header at the transmit side. This type of demodulation selection does not require a priori knowledge of the correlation of channel with modulation type. As will be appreciated, in different countries different channels are assigned to different types of service which makes this alternative method of ascertaining the channel modulation type attractive. The above, of course, requires that a modulation type flag be generated in the source coding at the transmit side.
In one example of the use of the universal modem in the ATV or advanced television transmission mode, assume that video data is to be transmitted in the MPEG format, where MPEG stands for Motion Picture Expert Group and where one utilizes the MPEG-2 format currently in vogue. Video information is source coded in MPEG in which 188 bytes of information are transmitted as a packet. Channel coding which defines the modulation format is imposed on top of the source coding to provide error correction which adds typically 20 more bytes of information to the data stream.
After the video information has been source coded and channel coded by the software-configurable modulator for a given format, it is ready for transmission. Note that prior to the transmission of the data, a header, e.g. a 3-bit header word: 000=VSB, 001=QAM, 010=QPSK, 011=DMT etc., can be inserted into the channel coding so as to provide an indication of the modulation type.
While the above has been described in terms of standardized formats for terrestrial, cable, satellite or telephone communications, the universal modem is also applicable to such diverse areas as cellular, PCS and other wireless services to provide automatic reconfiguring of a transceiver to permit its use across different formats. This is especially useful with cellular phone formats CDPD, CDMA, TDMA, FDMA and GSM referring respectively to cellular digital packet data, code division multiplexing, time-division multiplexing, frequency-division multiplexing, and the global mobile system type of modulation. While a cellular phone may be provided in one community for use with CDMA, when the phone roams to an area where TDMA transmissions are utilized, a universal modem within the phone can detect the change and reconfigure the demodulator side of the modem. Likewise, since the cellular system is a full duplex system, the universal modem can be reconfigured in the transmit mode to appropriately format the cellular transmission. It will of course be appreciated that the universal modem can be used with any type of RF transceiver roaming from one area to another where different formats are used, regardless of the type of service that is authorized.
Further, the universal modem is applicable to any kind of digital modulation method. For instance, it can be applied to digital terrestrial broadcasting utilizing the OFDM or Orthogonal Frequency Division Multiplexing method scheduled for use in Europe, Japan and/or other countries.
In one embodiment, the demodulation section of the modem, involves an analog-to-digital converter to provide a digital bit stream to a demodulator which is software-reconfigured in accordance with the output of a configuration RAM. In this embodiment, circuit blocks, elements or modules within the demodulator are linked up to provide a particular type of carrier recovery in accordance with the output of the configuration RAM. In another embodiment, the demodulator is layered, with each layer containing a different carrier recovery circuit. The configuration RAM can call for a given type of demodulation merely by activating one of the layers. This means that in any event, the configuration of the demodulator can be fixed by providing it with blocks, elements or modules that are connected in a particular way on the fly in response to the output of a configuration RAM under the control of a configuration controller. In a further embodiment, the software-configurable demodulator can be implemented in software using a general purpose microprocessor. Note that the above software reconfiguration techniques utilized for the demodulator can likewise be used for a software-configurable modulator.
For over-the-air and cable signals requiring equalization, the output of the configuration RAM may also be utilized to reconfigure an equalizer coupled to the output of the demodulator. Alternatively, when no follow-on signal processing is required, the only element that is to be reconfigured is the demodulator. It will be noted that equalizer are in general used to correct intersymbol interference, whereas in television applications, equalizers are used to cancel ghost images.
As to setting of the universal modem for modulation type or format as mentioned before, flags can be used on the transmission which are detected at the receive side. Alternatively, the channel to which the receiver is tuned can be detected. Moreover, a smart card or other similar device can be used to set the modulation type or format at the receive side. An additional way of detecting modulation type is to demodulate an incoming data stream using all possible formats, detecting the error and setting the demodulator to the modulation type having the lowest error.
In summary, a universal modem has a software-configurable modulator/demodulator which accommodates different modulation formats such as those associated with terrestrial, cable, phone line, satellite and wireless communications to be transmitted and received through a single device in which the modem has reconfigurable logic to accommodate the format of the signals being received or transmitted. Note that the modulator or demodulator can be used separately or the two units can be combined and used for transceivers, with either the same software configuring both the modulator and demodulator, or with different software used for the two units. In the receive mode, the universal modem detects the modulation format of the incoming signal and reconfigures the logic of its software-configurable demodulator to output demodulated digital data for further processing. In the transmit mode, information to be transmitted is provided with the appropriate modulation format by setting a software-configurable modulator in the universal modem to the particular format. In one embodiment, in the receive mode the incoming signal is A-D converted, with a host processor utilized to detect the type of modulation associated with the incoming signal and through a configuration controller configures a random access memory which is coupled to the software-configurable demodulator. In advanced television applications, the demodulated signal is passed through an equalizer, a Trellis Decoder and Reed-Solomon Decoder to provide an appropriate filtered and level-adjusted digital signal that is coupled to the next processing stage.